In recent years, environmental conservation and the recycling of resources are becoming the objects of public attention. Especially, municipal refuse and industrial waste are showing a yearly increase in quantity to such an extent that, within and outside large cities, it is becoming difficult to find out a new site to be filled up with these kinds of waste in a raw state. Therefore, it is common to incinerate these kinds of waste and reduce the volume thereof before taking them to a site to be filled up therewith.
However, there are many cases where municipal refuse and industrial waste contain metals and various kinds of harmful matter, which remain in the ashes and incombustibles (hereinafter called the "incineration residue") after incineration. Underground water will be polluted if a site is filled up with such incineration residue. Another trouble is that, since incineration residue has low specific gravity, an open pit to be filled up therewith must have a large capacity. Furthermore, soft ground results from incineration residue with which a site is filled up, and this soft ground hardly has utility value.
A previously proposed arrangement for eliminating these various difficulties is characterized in melting the incineration residue by a plasma arc and then cool it for solidification. Heavy metals such as chromium are sealed in the solidified slag and prevented from penetration therethrough outwardly thereof. Therefore, underground water is not polluted when a site is filled up with such slag. It will not only turn out to be a contribution to the recycling of resources to use this slag as aggregate to be mixed with concrete or as material for roadbed, but also relieve those skilled in the art of their anxiety about finding out a new site to be filled up with this slag.
For example, Japanese Laid Open Patent Application No. 3=55411 describes a plasma furnace for melting the incineration residue by a plasma arc. This plasma furnace includes a furnace body having a chamber for allowing molten slag to stay. The chamber is provided with an incineration residue inlet and a slag outlet. This plasma furnace further includes a plasma torch for striking a plasma arc. In operation, the incineration residue is fed through the incineration residue inlet and melted by the plasma arc. Molten slag is allowed to stay in the chamber of the furnace body, except that a portion of the molten slag is allowed to continuously leave the chamber through the slag outlet so as to be cooled and solidified. In case of an ordinary furnace designed for use in melting metals, it is most common to dispose a plasma torch at the center of the cylindrical furnace. In case of a plasma furnace described in British Patent Specification No. 1,390,351/3, the plasma torch is swung about its pivot disposed at the center of the furnace. In case of a plasma furnace described on pages 170 and 171 of the Research Paper Vol. 41, No. 2, 1985 published by Tohoku University Dressing and Smelting Laboratory, the plasma torch remains tilted while it revolves round the center of the furnace so that a large area within the furnace designed for use in the direct reduction of chromite ores may be uniformly heated.
Incineration residue to be melted in a plasma furnace of the kind indicated above has a grain size of several microns at the most. Dispersion of pulverulent incineration residue into ambient air is apt to occur when such incineration residue is being fed to the furnace or is about to be melted in the furnace. In order to confine any such dispersion, it is most common to place the incineration residue under the influence of negative pressure caused by an induced draft fan in the direction of flow of the incineration residue through the furnace.
Waste gas discharged from a plasma furnace has a temperature approximating 1300.degree. C. Therefore, a flue to be passed by this waste gas is lined with refractory material. After passage through this flue, the waste gas is quenched in a cooler such as a water spray cooling chamber, allowed to pass through a dust catcher, and discharged into the open air. There are some cases where a heat exchanger is connected to the flue for the recovery of waste heat.
FIGS. 22(a) and 22(b) are sectional views to help explain the structural construction of plasma torches generally used in plasma furnaces of the kind indicated above. Each of these plasma torches comprises an anode 651, a cathode 652, a water jacket 653 and a plasma gas inlet 655. These plasma torches are intended for two modes of operation respectively, the difference therebetween being derived from the difference in where the cathode is disposed. One of the two modes of operation, i.e. the nontransfer mode, is shown in FIG. 22(a), in which a plasma arc 654 is struck between the anode 651 disposed in the plasma torch and the cathode 652 disposed on the lower end of the plasma torch. The other of the two modes of operation, i.e. the transfer mode, is shown in FIG. 22(b), in which the plasma arc 654 is struck between the anode 651 disposed in the plasma torch and the cathode 652 disposed on a furnace body. A plasma gas supply pipe (not shown) is connected to the plasma gas inlet 655.
In case of the former mode of operation, the plasma arc is struck between two portions of the plasma torch. In case of the latter mode of operation, two portions between which the plasma arc is struck are disposed on the furnace body and the plasma torch respectively. Plasma is an ionized gas having a temperature ranging from 3,000 to 10,000K. This temperature is high enough to cause damage to the plasma torch. The severest damage is done to the above-mentioned two portions, while the second severest damage is done to a portion of the plasma torch disposed medially of these two portions. Therefore, the provision of the water jacket 553 results from priority given to the anode 651 and its vicinity in providing a means for preventing the plasma torch from sustaining damage caused by intense heat.
A plasma arc having a temperature up to 10,000K facilitates the decomposition of harmful matter such as dioxine. A small-sized waste-gas purifying plant meets the need of a plasma furnace, because the amount of waste gas discharged from the plasma furnace is less than one-thirtieth of the amount of waste gas discharged from a combustion furnace.
However, each of the prior art plasma furnaces has the disadvantages that, when it is used for melting the waste, the slag outlet is apt to be choked up, that a portion of the waste is discharged through the slag outlet in an unmelted state, and that waste gas discharged from the prior art plasma furnace contains nitrogen oxides (NO.sub.x) and heavy metals. These disadvantages pose the following problems: